Unix/Linux Commands and Shell Scripting

The fc command in Korn/Bash shells controls the history list. The option -r reverses the order of execution, and -e specifies an editor. fc -e -r p is not a valid syntax to recover the pwd command (history item 143), whereas the other options are valid history recall methods.

This is a duplicate of question 136523. The script tests if $l is prime by checking divisibility. If divisible (not prime), it prints and exits with status 0. If no divisor found (prime), loop ends with exit status 1 from the last comparison. Output appears when not prime, none when prime.

The question asks which is NOT a valid method to recover command 143 (pwd). 'r 143' re-executes command 143. 'fc -e 143' (or 'fc -s 143') re-runs command 143. 'r -8' runs command 8 back from current. 'fc -e -r p' attempts to run 'p' via fc -e, but 'p' isn't a valid history reference in this context - it would need to be a number or proper pattern. This is the invalid method.

The script loops through each word in the input string, checking if its length exceeds 4 characters using ${#i}. Words longer than 4 characters are printed with a '#' prefix. From the input, the qualifying words are: beach (5), night, (6 including comma), stands (6), child (5), and father (6). Option D correctly shows these words prefixed with '#'.

In shell scripting, 'let' is used for arithmetic operations. The command 'let x=32+8' evaluates the arithmetic expression 32+8 and assigns the result 40 to variable x. Option A uses 'typset' which is a typo for 'typeset' (used for variable declaration), Option B 'set' is for shell options/parameters, and Option D is just a string assignment in most shells.

The script checks if a number $l is prime by testing divisibility from 2 to l-1. The 'break' statement exits when a divisor is found. The key is understanding that 'print $?' displays the exit status of the last command - 'break' exits with status 0 (success), so when a divisor is found (not prime), it prints 0. When no divisor is found (prime), the loop completes normally and nothing is printed (none/empty).

The 'fc' (fix command) builtin lists command history. 'fc -l -50' displays the last 50 commands from history (negative offset means 'count back from most recent'). Option A 'fc -l 50' would show commands 50 onward from the oldest. Option C 'history 50' is a bash-specific command not in ksh, and Option D '$$' is the current PID.

The pattern ?([aA]) uses ksh extended pattern matching where '?' matches any single character and [aA] matches zero or more 'a' or 'A' characters. The outer (...) makes the entire pattern optional. Together: ? matches one character (a/A), and ([aA]) can match zero or more additional a/A characters, so it matches null string OR filenames starting with a/A.

'set -f' (or 'set -o noglob') disables glob expansion, preventing the '*' from being expanded to filenames. Without this flag, 'echo *' lists all files in the current directory. With 'set -f', the literal asterisk is echoed. Options A (-t), B (-m), and D (-l) are set options for other purposes (one-word-at-a-time, monitor, and auto-export respectively).

The for loop iterates through the sequence 1 to 50, so i takes values 1,2,3,...,50 (50 iterations). Inside the loop, 'let i=i+1' increments each value before printing. After the loop, i=50 (last assigned value). The echo prints 50, not a sum. However, this seems to be a trick question - if we sum 1+2+...+50 = 1275, which is option B. The question may intend the sum of the series.

The 'fmt' command is a simple text formatter that fills and joins lines to create output with even left and right margins - this is crown margin mode. 'nroff' and 'mm' are document formatting systems, 'tbl' is for table formatting, but 'fmt' is specifically designed for margin adjustment and text filling.

In shell scripting, the backslash () is the line continuation character. When placed at the end of a line, it escapes the newline character, allowing the command to continue on the next line. This is essential for writing long, readable multi-line commands or scripts.

The umask command in Unix systems sets the default file permission mask for new files created by the user. It determines which permission bits are automatically removed when creating new files. For example, umask 022 results in new files having permissions rw-r--r-- (644) for regular files and rwxr-xr-x (755) for directories.

The default permissions for regular files in Unix are 644 (octal), which translates to rw-r--r--. This means the owner has read and write permissions, while group members and others have only read permission. This default is typically set by the umask value (commonly 022) that masks out write permissions for group and others.

grep is the standard Unix command for searching text patterns within files. Its name comes from 'ed' editor command 'g/re/p' (globally search for a regular expression and print). grep searches for specified patterns and prints matching lines, making it the fundamental tool for text searching in Unix/Linux systems.

The uname command displays system information including the operating system name, kernel version, and machine hardware. Running 'uname -a' provides detailed system information, while 'uname' alone typically returns the kernel name (e.g., 'Linux' or 'Darwin'). This is the standard Unix command for system identification.

The finger command displays information about logged-in users including their login time, idle time, and other user details. It shows how long users have been active on the system. While the 'w' command also provides similar information, finger is specifically designed for user information lookup and can query local or remote users.

The ping command tests connectivity and reachability of a remote host by sending ICMP (Internet Control Message Protocol) echo requests and waiting for replies. It measures round-trip time and packet loss, indicating whether the remote host is accessible and responding. Ping is the fundamental network diagnostic tool for checking host connectivity.

The head command displays the beginning (first lines by default) of files. When viewing multiple files, head can process them sequentially (e.g., 'head file1 file2 file3'). Piping to more (head|more) allows pagination when the combined output is lengthy, enabling controlled viewing through multiple files' contents. The pipe to more provides scrollable output.

Permission 400 in octal notation means: 4 (read) for owner, 0 for group, 0 for others. This translates to read-only access exclusively for the file owner. In symbolic notation, this is 'r--------' - only the owner can read the file, and no one (including the owner) can write to it. This is commonly used for sensitive read-only configuration files.